1. Field of the Invention
The present invention relates to a liquid drop discharging head and a liquid drop discharging device and, in particular, to a liquid drop discharging head that discharges liquid drops to record letters and images on a recording medium or to form fine patterns and thin films on a substrate and a liquid drop discharging device provided with this liquid drop discharging head.
2. Description of the Related Art
A liquid drop discharging method has been generally well known for generating a pressure wave (acoustic wave) in a liquid filled in a pressure developing chamber by pressure developing means such as a piezoelectric actuator and for discharging liquid drops from nozzles connected to the pressure developing chamber by the pressure wave. In particular, an ink jet recording device has been widely used that discharges drops of ink to record letters and images on recording paper (for example, Japanese Patent Application Publication (JP-B) No. 53-12138 and Japanese Patent Application Laid-Open (JP-A) No. 10-193587). In recent years, an image of extremely high quality can be recorded by reducing the volume of a drop of ink and by the use of ink of a low concentration.
Moreover, in recent years has been tried an industrial application of a liquid drop discharging device using the above liquid drop discharging method. Main applications include:
(a) an electrically conducting polymer solution is discharged onto a substrate to form a wiring pattern and a transistor;
(b) an organic EL solution is discharged onto a substrate to form an EL display panel;
(c) fused solder is discharged onto a substrate to form electrical mounting bumps;
(d) liquid drops of UV cure resin or the like are laminated and cured on a substrate to form a three-dimensional body; and
(e) an organic material solution (resist solution or the like) is discharged onto a substrate to form an organic thin film.
In this manner, the liquid drop discharging device has been utilized not only in recording images but also in extensive fields. It is expected that the liquid drop discharging device will be utilized in more extensive fields in the future.
Incidentally, in the following, an object onto which liquid drops are discharged from a liquid drop discharging head is called “a recording medium” and a dot pattern on a recording medium formed by the liquid drops adhering to the recording medium is called “an image” or “a recorded image”. Therefore, “the recording medium” in the following description includes not only a recording sheet and an OHP sheet but also, for example, the substrate described above and the like. Moreover, “the image” in the following description includes not only a general image (letter, picture, photograph), but also the wiring pattern, the three-dimensional body, the organic thin film, which have been described above, and the like.
An example of a liquid drop discharging mechanism (ejector) in a liquid drop discharging device publicly known in the above patent gazette or the like is shown in a cross-sectional view in FIG. 13. A nozzle 16 for discharging a liquid drop and a supply passage 20 for guiding liquid from a liquid tank (not shown) through a common passage 18 are connected to a pressure developing chamber 14. Moreover, a vibration plate 22 is fixed to the bottom of the pressure developing chamber 14. When a liquid drop is discharged, the vibration plate 22 is displaced by a piezoelectric actuator 24 mounted on an opposite side of the pressure developing chamber 14 with the vibration plate 22 sandwiched between them to change the volume of the pressure developing chamber 14 thereby to develop a pressure wave. This pressure wave ejects out a part of liquid filled in the pressure developing chamber 14 through the nozzle 16 to fly a liquid drop 26. The flied liquid drop 26 attaches to a recording medium such as recording paper and forms a dot (pixel). By repeating the formation of the dot in this manner based on image data or the like, a pattern such as a letter, an image or the like is recorded (formed) on the recording medium.
In the liquid drop discharging device described above, it is an improvement in a recording speed that presents a significant challenge at present. In the liquid drop discharging device, the largest parameter affecting the recording speed is the number of nozzles and as the number of nozzles increases, the number of dots to be formed in a unit time increases and the recording speed increases. For this reason, in an ordinary liquid drop discharging device, a multi-nozzle type liquid drop discharging head (linear array head) is widely employed in which a plurality of ejectors are connected to each other.
A linear array head 32 is shown in FIG. 14 as an example of the multi-nozzle type liquid drop discharging head. In this linear array head 32, a liquid tank (not shown) is connected to a common passage 36 through a liquid supply port 34 and a plurality of ejectors 38 are connected to this common passage 36.
However, in a structure in which the ejectors 38 are arranged one-dimensionally (linearly), the number of ejectors cannot be so much increased (usually, the maximum number of ejectors is about 100).
Then, some liquid drop discharging heads have been proposed until now in which the number of ejectors is increased by two-dimensionally arranging the ejectors in the form of matrix (hereinafter referred to as “matrix array head)(JP-A Nos. 1-208146 and 9-156095).
Examples of basic structure of a conventional matrix array head are shown in FIGS. 15A and 16A, respectively.
In these matrix array heads 42 and 52, a plurality of ejectors 44 are connected to each common passage 46 and further a plurality of common passages 46 are connected to a second common passage 48. For example, in the matrix array head 42 shown in FIG. 15A, the common passages 46 are arranged along a main scanning direction (shown by an arrow M) of the head and the second common passage 48 is arranged along a direction perpendicular to the main scanning direction (sub-scanning direction, shown by an arrow S). The respective ejectors (44A to 44H) connected to the same common passage 46 are shifted by pitches Pn in the sub-scanning direction. In a process of scanning the head in the main scanning direction, by discharging liquid drops from the respective ejectors while controlling a discharging timing, dots 50 are formed at the pitches shown in FIG. 15B.
On the other hand, in the matrix array head 52 shown in FIG. 16A, the common passages 46 are arranged along the sub-scanning direction (shown by an arrow) of the head and the second common passage 48 is arranged along the main scanning direction. Also in this case, the respective ejectors arranged adjacent to each other are shifted by the pitches Pn in the sub-scanning direction. In a process of scanning the head in the main scanning direction, by discharging liquid drops from the respective ejectors while controlling a discharging timing, dots 50 are formed at the pitches Pn shown in FIG. 16B.
The matrix array head having such a structure is very advantageous to recording an image at high speeds because the number of ejectors can be increased. For example, in the matrix array head 42 shown in FIG. 15A, if the number of common passages 46 is 26 and 10 ejectors 44 are connected to each of the common passages 46, 260 ejectors can be arranged (in FIG. 15A, the number of common passages 46 is 8 and 8 ejectors are connected to one common passage 46 and hence only a total of 64 ejectors 44 are shown).
However, the conventional matrix array head described above is advantageous to a high-speed recording, whereas it presents a problem that it is difficult to provide high uniformity in a recorded result. To be specific, the conventional matrix array head raises a problem that it tends to produce cyclical variations in a print density (variations in dot diameter) in a direction perpendicular to the main scanning direction of the head (sub-scanning direction) and hence significantly impairs uniformity in the recorded result.
Although the reason why such variations in the print density are easily caused in the matrix array head is variously considered, in many cases, the variations in the print density are particularly caused by the fact that the discharging characteristics (volume and speed of the liquid drop) of the ejector tend to vary according to the positions where the ejectors are connected to the common passage.
That is, in the matrix array head, the respective ejectors are connected to a long slender common passage, so that the characteristics (passage resistance and inertance) of the common passage when viewed from the respective ejectors vary according to the positions where the ejectors are connected to the common passage. For example, in FIG. 15A, the effective length (Lc) of the common passage becomes small for the ejector 44A connected to the base portion of the common passage 46, so that the passage resistance and inertance of the common passage 46 also become small (the passage resistance and the inertance are proportional to a passage length). On the other hand, for the ejector 44H connected to the tip portion of the common passage 46, the effective length (Lc′) of the common passage becomes large, so that the passage resistance and inertance of the common passage 46 also become large. The passage resistance and inertance of the common passage 46 significantly affects the refill characteristics (which will be described later) of the respective ejectors and, as a result, change discharging characteristics (volume and speed of the liquid drop) of the respective ejectors 44. For this reason, differences are produced in the discharging characteristics between the respective ejectors 44, depending on the positions where the ejectors are connected to the common passage 46.
In FIG. 15B is schematically shown an effect that the above-mentioned differences in the discharging characteristics between the ejectors have on the uniformity in the recorded result. Here, description will be made in the following on the assumption that the ejector connected to the base portion of the common passage 46 has a large liquid drop volume (dot diameter) and the ejectors connected to the portions nearer to the tip of the common passage 46 have smaller liquid drop volumes (dot diameters), which is a tendency generally observed in this matrix array head. (However, depending on the passage resistance and inertance of the common passage, there are cases where the ejector connected to the base portion of the common passage 46 has a small liquid drop volume (dot diameter) and the ejectors connected to the portions nearer to the tip of the common passage 46 have larger liquid drop volumes (dot diameters). Further, there are cases where the liquid drop volume (dot diameter) has a complex tendency, for example, the liquid drop volume (dot diameter) decreases or increases as the positions of the ejectors come nearer to the both ends (base portion and tip portion) from the center of the common passage 46).
In a case where there is the above-mentioned difference (distribution) in the liquid drop volume, in a line of recorded dots, as shown in FIG. 15B, the dot diameter changes in a cycle of n (where n is the number of ejectors connected to one common passage 46 and in the case shown in FIG. 15b, n=8). In short, variations in the print density having a cycle of n are caused in the sub-scanning direction in the recorded result. In the general matrix array head, n is set at about 4 to 20 and a recording resolution in the sub-scanning direction is set at about 150 to 600 dpi (dot/inch) and hence the cycle of the above-mentioned variations in the print density become about 0.17 to 3.4 mm. That is, the general matrix array head causes the variations in the print density having a space frequency of 0.3 to 5.9 cycle/mm.
In FIG. 17, human eye's sensitivity to variations in the print density is shown in a graph with a horizontal axis as the space frequency. It can be found from this graph that when the space frequency of variations in the print density is 6 or less cycle/mm, the human eye's sensitivity to variations in the print density increases and human eyes can easily sense variations in the print density. In particular, in a case where the space frequency is not larger than 3 cycle/mm, the human eye can extremely easily sense variations in the print density. Here, for the space frequency not larger than 1 cycle/mm, there exist both of data (broken line) showing that the sensitivity decreases and data (solid line) showing the sensitivity does not decrease, but according to experimental results obtained by the inventors, it is said that the data shown by the solid line well express the actual sensitivity of the human eyes.
With consideration given to human eye's characteristics, variations in the print density having a space frequency of 0.3 to 5.9 cycle/mm caused by the conventional matrix array head are those very easily sensed by the human eyes, which results in significantly impairing the quality of the recorded result. In order to make the human eyes become hard to sense variations in the print density, it is necessary that the space frequency of variations in the print density be set at 6 or more cycle/mm, preferably, 10 or more cycle/mm. However, by the conventional multi-nozzle array head, it is difficult to realize this space frequency and thus it is impossible to perform a highly uniform recording.
Moreover, even in a case where the passage arrangement shown in FIG. 16A, there is presented a problem that variations in the print density are caused by the positions where the ejectors are connected to the common passage. In a case where such passage arrangement is employed, the cycle of variations in the print density becomes a head length (LH) in the sub-scanning direction and hence variations in the print density become very large. For example, in a case where a recording resolution in the sub-scanning direction is 300 dpi and the number of ejectors is 260, the head length in the sub-scanning direction becomes about 22 mm and hence the cycle of variations in the print density becomes about 22 mm (the space frequency becomes about 0.05 cycle/mm). Variations in the print density having such a low frequency are also very easily sensed by the human eyes, thereby impairing the uniformity of the recorded result.
As described above, in the conventional matrix array head, variations in the print density tends to be caused in the direction perpendicular to the main scanning direction of the head (sub-scanning direction) by the difference in the discharging characteristics between the respective ejectors. These variations in the print density become noticeable particularly in a case where the ejectors are to be arranged at high density. This is because since the width of the common passage is required to be set very small so as to increase the arrangement density of the ejectors, the passage resistance and the inertance of the common passage increase, which results in inevitably increasing the differences in the discharging characteristics between the respective ejectors that are caused by the positions where the ejectors are connected to the common passage. In other words, as the number of nozzles (nozzle density) is increased so as to increase a recording speed, the quality of recorded result tends to be degraded and hence it is extremely difficult to realize compatibility between high-speed recording and high-quality recording.
Here, in JP-B No. 10-508808 is disclosed the matrix array head 62 shown in FIG. 18.
In this matrix array head 62, passages 64 correspond to the common passages 46 shown in FIG. 15A. The passages 64 are arranged along the direction perpendicular to the main scanning direction M of the matrix array head 62 (sub-scanning direction S). Moreover, passages 66 corresponding to the second common passage 48 shown in FIG. 15A are arranged at two portions of the top and bottom portions of a group of ejectors 70 constructed of a plurality of ejectors 68. The passages 64 connected to each of the passages 66 are arranged alternately in the main scanning direction. The respective ejectors 68 are connected to each other through two adjacent passages 64 and a supply passage 72. With this method of arranging the passages 64 and this method of connecting ejectors 68, it is possible to prevent the occurrence of the above-mentioned variations in the print density and hence to perform highly uniform recording.
However, in a case of this matrix array head 62, there is presented a problem that since the common passages (passages 64) need to be arranged in such a way as to pass through the group of ejectors 70 in the sub-scanning direction, the length of the group of ejectors cannot be elongated in the sub-scanning direction and hence this matrix array head 62 cannot respond to high-speed recording. That is, if the ejectors 68 are increased in number so as to realize high-speed recording, the length of the group of ejectors (head length) is increased in the sub-scanning direction and hence the total length of the common passages (passages 64) is very much increased. As a result, the passage resistance of the passage 64 is very much increased, so that even if the passage arrangement shown in FIG. 18 is used, it is impossible to realize highly uniform recording (or presents a problem of increasing the size of the head).